The present invention relates generally to the production of oxygen gas (xe2x80x9cGOXxe2x80x9d) and, in particular, to the production of low pressure GOX by the cryogenic distillation of air.
There is a considerable market, particularly in the glass and metallurgical industries, for low purity, e.g. from 90 to 98 wt %, low pressure, e.g. from 1.5 to 3.0 bar absolute, GOX. The GOX is used in processes requiring oxygen-enriched combustion in which the required pressure of the oxygen at the point of use is near atmospheric.
An O2 vacuum swing absorption (xe2x80x9cVSAxe2x80x9d) process is commonly used for applications requiring 90 to 93% O2. However, up to 98% O2 GOX is often required and, thus, cryogenic plant processes are also used. There are many prior public disclosures of processes using cryogenic distillation of air to produce a GOX product. A number of the disclosed processes use a liquid cryogen from an external source as a refrigerant. For example, in U.S. Pat. No. 4,853,015 (Yoshino) and U.S. Pat. No. 4,732,595 (Yoshino), liquid oxygen (xe2x80x9cLOXxe2x80x9d) is injected into the low pressure column of a double column distillation system to provide refrigeration. In U.S. Pat. No. 4,732,595, an expander is used to provide some of the refrigeration requirement of the process. The use of such an expander increases the overall capital and running costs of the process and, as such, is undesirable.
In U.S. Pat. No. 5,408,831 (Guillard et al), air is distilled cryogenically in a double distillation column system without the use of an expander to provide a portion of the refrigeration duty. GOX at from 2 to 5 bar absolute is taken from the low pressure (xe2x80x9cLPxe2x80x9d) column of the distillation column system as product. It is an essential feature of the Guillard process that some refrigeration is provided by expansion of at least one gaseous product from an LP column of the distillation column system. Part of the refrigeration duty required to condense the feed air fed to the column system can be provided by LOX refrigerant from an external source. The refrigerant may be introduced into the LP column or into the GOX product at an intermediate location of the main heat exchanger. The actual temperature at which the LOX is introduced is chosen to minimise the risk of explosion of any hydrocarbon impurities.
In order to achieve the required pressure of the GOX product, the column system in U.S. Pat. No. 5,408,831 is back-pressurized. As a result of the back-pressurization of the column system, the air pressure is necessarily higher, at a pressure from 8 to 16 bar absolute, than that in processes without column back-pressurization giving a significant power penalty of about 12% for a given air flow. Such a penalty represents an undesirable increase in operating cost especially when it is considered that power is the main operating cost of an air separation plant.
Processes producing low pressure GOX in which air compressor power is minimized without adversely effecting both the overall capital and running costs are desirable. In this connection, it is known to provide at least part of the refrigeration duty required to cool and at least partially condense by heat exchange feed air prior to cryogenic distillation through the use of a LOX refrigerant from an external source.
U.S. Pat. No. 5,505,052 (Ekins et al) discloses a process for the cryogenic distillation of air using a double column system having a high pressure (xe2x80x9cHPxe2x80x9d) column and a LP column to produce GOX at a pressure of about 25 bar for use in installations comprising, for example, electric arc furnaces adapted to produce stainless steel. Oxygen is withdrawn in liquid form from the base of the LP column, brought to the utilization pressure by a pump and vaporized and reheated to about ambient temperature in the heat exchange line against the feed air. The gaseous oxygen is then fed to the installation.
A portion of the LOX withdrawn from the base of the column may be sent to storage, for example, during periods of low demand for GOX in the installation where it is kept until such time as the demand for GOX at the installation becomes high whereupon it is pumped to the utilization pressure and vaporized and reheated to about ambient temperature in the heat exchange line against the feed air. The LOX from storage may travel through the same vaporization passages through the heat exchange line as the LOX from the column system or it may travel through separate vaporization passages. The gaseous oxygen is then fed to the installation.
Additional LOX may be added to the LOX in storage from tank trucks, for example, during prolonged periods of high demand for GOX. Alternatively, the storage facility may not be connected to the double column system and may be supplied only by tank trucks. In the exemplified embodiments of the process disclosed in Ekins et al, LOX, whether from the double column system or from storage, enters the heat exchange line at the cold end, i.e. the end at which cooled feed air exits the line.
In Ekins et al, both the LOX product from the distillation column and the additional LOX from storage are pumped to a pressure (about 25 bar) that is substantially higher that the pressure (about 5 to 6 bar) of the LP column. In addition, a portion of the total refrigeration duty requirement of the process is provided by an expander and a further portion is provided by the warming and evaporation of a stream of liquid argon.
It is an objective of the present invention to provide a process and apparatus for the production of low pressure GOX with lower capital and operating costs compared with existing processes. It is a further objective that the process reduce the risk of explosion resulting from deposition in the heat exchange line of impurities, for example hydrocarbons, CO2 and N2O, from LOX. This risk is explained in more detail in the xe2x80x9cDetailed Description of the Inventionxe2x80x9d section below.
It has been found that the objectives of the invention can be achieved using a process in which a portion of the refrigeration duty is provided by LOX refrigerant from an external source. According to a first aspect of the present invention, there is provided an improved process for the production of GOX, said process comprising:
cooling and at least partially condensing feed air by heat exchange using heat exchange means having a warm end and a cold end to produce cooled and at least partially condensed feed air;
distilling said cooled and at least partially condensed feed air in a distillation column system to produce LOX product;
removing a stream of said LOX product from the distillation column system and vaporizing said LOX product stream by heat exchange against the feed air to produce GOX; and
separately from the LOX product, vaporizing LOX refrigerant from an external source by heat exchange against the feed air to produce vaporized refrigerant thereby providing a portion of the refrigeration duty required to cool and at least partially condense the feed air;
the improvement consisting of injecting the LOX refrigerant into the heat exchange means at a pressure greater than that of the LOX product entering the heat exchange means.
In accordance with the first aspect of the present invention, there is also provided an improved process for the production of GOX, said process comprising:
cooling and at least partially condensing feed air by heat exchange using heat exchange means having a warm end and a cold end to produce cooled and at least partially condensed feed air;
distilling said cooled and at least partially condensed feed air in a distillation column system to produce LOX product;
removing a stream of said LOX product from the distillation column system and vaporizing said LOX product stream by heat exchange against the feed air to produce GOX; and
separately from the LOX product, vaporizing LOX refrigerant from an external source by heat exchange against the feed air to produce vaporized refrigerant thereby providing a portion of the refrigeration duty required to cool and at least partially condense the feed air;
the improvement consisting of injecting the LOX refrigerant into the heat exchange means at a pressure substantially equal to that of the LOX product entering the heat exchange means and at an intermediate point between the warm and cold ends where the temperature of the heat exchange means is above the boiling temperature of the LOX refrigerant.
According to a second aspect of the present invention, there is provided apparatus for carrying out the process of the first aspect of the present invention for producing gaseous oxygen, said apparatus comprising:
heat exchange means for cooling and at least partially condensing feed air to produce cooled and at least partially condensed feed air, said heat exchange means having a warm end and a cold end;
a distillation column system for distilling cooled and at least partially condensed feed air to produce LOX product;
conduit means to carry cooled feed air from the heat exchange means to the distillation column system; and
conduit means to carry LOX product from the distillation column system to the heat exchange means;
wherein the apparatus further comprises conduit means to carry LOX refrigerant at a greater pressure than the pressure of the LOX product entering the heat exchange means from an external supply to the heat exchange means.
(b) conduit means to carry LOX refrigerant at a pressure that is substantially equal to the pressure of the LOX product entering the heat exchange means from an external supply to an intermediate point between the warm and cold ends of the heat exchange means where the temperature of the heat exchange means is above the boiling temperature of the LOX refrigerant.